How to quickly turn on/off N-FET switching high side of 55V?

[Winfield Hill]

Winfield Hill wrote:
John Fields wrote:
Here's my second cut for the 55V 30A high side driver:
Version 4 [ snip ]
I'm not sure how useful it is to post Spice analysis
files, it tends to stop or slow down the conversation,
because one can't quickly read it on the spot.
---
I like ASCII as much as anyone, but when stuff starts getting
complex to the point where timing is an issue or an ASCII drawing
would just be too busy, I prefer to either run a simulation and post
the schematic list or post a PDF to abse. In any case, how much
bother could it be, if one is interested, to post the thing into
LTSPICE and print out the schematic?

Sorry, but it probably limits the conversation to a
few -- you, me, anybody else? And look at the delay.
You may not have had any answer at all.

---
Well, Win, the design was for the OP so I'd expect that he, at
least, would be willing to accept a format that was convenient for
me to work in and to provide a working model of the circuit.

Whether it's inconvenient for you is really immaterial. Play
my way if you want to, else why don't you ASCII-ize the drawing
for everyone else who you think wants to play your way?

NOW, who's being bitchy?

It's bad, especially if you modify the spice BJT part
model to show the MAXIMUM beta, rather than the minimum
as is usually done in the factory models.

Anyway, thanks for pointing that error out so politely and, if you
can spare the time, see the file at the end of this post for the
easy fix.

Yes, well done.

---

The driver's currents should be referenced to the
MOSFET's source. That's where the capacitance is.

---
I prefer to reference the currents to ground for a few reasons:

1. With the load looking like a couple of ohms it won't make any
difference in how long it takes to charge or discharge the gate
capacitance and,

2. The source jumps up to about 50V if the load is disconnected or
opens (whether the switch is on or off) and,

3. With the switch turned off, any current in the driver will have
to return to ground through the load. Not exactly my cup of tea,
but YMMV.
---

In the noise level, compared to the mayhem underway.

---
My, but you're in a bitchy mood.

Maybe this'll cheer you up:

Version 4 [ snip '

OK, OK, that's MUCH better John. I'll give you
a pass on that version. But I won't make your
ASCII drawing for you. I have work to do!

 

[John Fields]

On Tue, 18 Dec 2007 09:56:49 -0600, John Fields

..
..
..

Snipped high side switch Rev B

Rev C:

Version 4
SHEET 1 1996 1156
WIRE 752 400 576 400
WIRE 880 400 752 400
WIRE 1104 400 880 400
WIRE 1216 400 1104 400
WIRE 1440 400 1216 400
WIRE 1216 432 1216 400
WIRE 576 480 576 400
WIRE 752 480 752 400
WIRE 880 480 880 400
WIRE 1104 480 1104 400
WIRE 1440 480 1440 400
WIRE 1216 528 1216 496
WIRE 1264 528 1216 528
WIRE 1376 528 1344 528
WIRE 752 608 752 544
WIRE 880 608 880 544
WIRE 880 608 752 608
WIRE 1648 608 880 608
WIRE 1216 624 1216 528
WIRE 1440 640 1440 576
WIRE 576 656 576 544
WIRE 624 656 576 656
WIRE 752 656 752 608
WIRE 752 656 704 656
WIRE 1648 656 1648 608
WIRE 576 704 576 656
WIRE 1104 704 1104 560
WIRE 1216 704 1216 688
WIRE 1216 704 1104 704
WIRE 208 736 96 736
WIRE 1216 736 1216 704
WIRE 1440 736 1440 720
WIRE 1600 736 1440 736
WIRE 368 752 272 752
WIRE 400 752 368 752
WIRE 512 752 480 752
WIRE 1104 768 1104 704
WIRE 1440 800 1440 736
WIRE 1504 800 1440 800
WIRE 1648 800 1648 752
WIRE 1648 800 1568 800
WIRE 96 816 96 736
WIRE 128 816 96 816
WIRE 240 816 192 816
WIRE 368 816 368 752
WIRE 368 816 320 816
WIRE 928 816 880 816
WIRE 1040 816 1008 816
WIRE 1440 848 1440 800
WIRE 1216 896 1216 800
WIRE 1264 896 1216 896
WIRE 1376 896 1344 896
WIRE 1648 912 1648 800
WIRE 96 928 96 816
WIRE 128 928 96 928
WIRE 240 928 192 928
WIRE 368 928 368 816
WIRE 368 928 320 928
WIRE 752 928 752 656
WIRE 880 928 880 816
WIRE 1216 928 1216 896
WIRE 96 960 96 928
WIRE 96 1040 96 1024
WIRE 576 1040 576 800
WIRE 576 1040 96 1040
WIRE 752 1040 752 1008
WIRE 752 1040 576 1040
WIRE 880 1040 880 1008
WIRE 880 1040 752 1040
WIRE 1104 1040 1104 864
WIRE 1104 1040 880 1040
WIRE 1216 1040 1216 992
WIRE 1216 1040 1104 1040
WIRE 1440 1040 1440 944
WIRE 1440 1040 1216 1040
WIRE 1648 1040 1648 992
WIRE 1648 1040 1440 1040
WIRE 96 1088 96 1040
FLAG 96 1088 0
SYMBOL pnp 1376 576 M180
WINDOW 0 75 83 Left 0
WINDOW 3 53 48 Left 0
SYMATTR InstName Q1
SYMATTR Value 2N5401
SYMBOL npn 1040 768 R0
WINDOW 0 71 15 Left 0
WINDOW 3 45 48 Left 0
SYMATTR InstName Q2
SYMATTR Value 2N5550
SYMBOL voltage 752 912 R0
WINDOW 3 40 81 Left 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
WINDOW 0 40 53 Left 0
SYMATTR Value 55
SYMATTR InstName V2
SYMBOL res 1632 896 R0
WINDOW 0 -39 37 Left 0
WINDOW 3 -41 68 Left 0
SYMATTR InstName R4
SYMATTR Value 1.8
SYMBOL voltage 880 912 R0
WINDOW 3 24 104 Invisible 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
WINDOW 0 39 57 Left 0
SYMATTR Value PULSE(0 5 0.02 1e-6 1e-6 0.05 0.1 1)
SYMATTR InstName V3
SYMBOL res 1024 800 R90
WINDOW 0 -46 56 VBottom 0
WINDOW 3 -38 58 VTop 0
SYMATTR InstName R9
SYMATTR Value 1000
SYMBOL nmos 1600 656 R0
SYMATTR InstName M2
SYMATTR Value SUM75N06-09L
SYMBOL res 1088 464 R0
SYMATTR InstName R2
SYMATTR Value 10k
SYMBOL res 1360 512 R90
WINDOW 0 68 48 VBottom 0
WINDOW 3 66 47 VTop 0
SYMATTR InstName R1
SYMATTR Value 50k
SYMBOL npn 1376 848 R0
WINDOW 0 72 17 Left 0
WINDOW 3 47 50 Left 0
SYMATTR InstName Q3
SYMATTR Value 2N5550
SYMBOL res 1360 880 R90
WINDOW 0 -25 58 VBottom 0
WINDOW 3 -27 56 VTop 0
SYMATTR InstName R3
SYMATTR Value 50k
SYMBOL res 1424 624 R0
WINDOW 0 41 43 Left 0
WINDOW 3 34 74 Left 0
SYMATTR InstName R5
SYMATTR Value 1000
SYMBOL ind 608 672 R270
WINDOW 0 45 56 VTop 0
WINDOW 3 -8 54 VBottom 0
SYMATTR InstName L1
SYMATTR Value 5e-3
SYMBOL npn 512 704 R0
SYMATTR InstName Q4
SYMATTR Value 2N5550
SYMBOL res 496 736 R90
WINDOW 0 -11 55 VBottom 0
WINDOW 3 35 55 VTop 0
SYMATTR InstName R6
SYMATTR Value 1000
SYMBOL diode 592 544 R180
WINDOW 0 51 34 Left 0
WINDOW 3 24 0 Left 0
SYMATTR InstName D1
SYMATTR Value MURS120
SYMBOL zener 768 544 R180
WINDOW 0 63 33 Left 0
WINDOW 3 24 0 Left 0
SYMATTR InstName D2
SYMATTR Value BZX84C10L
SYMBOL cap 864 480 R0
WINDOW 0 48 32 Left 0
WINDOW 3 28 61 Left 0
SYMATTR InstName C1
SYMATTR Value 10e-6
SYMBOL Digital\\schmitt 208 672 R0
SYMATTR InstName A1
SYMATTR SpiceLine vhigh 5 vh 1 trise 10e-9 tfall 10e-9
SYMBOL cap 80 960 R0
WINDOW 0 -44 33 Left 0
WINDOW 3 -51 64 Left 0
SYMATTR InstName C3
SYMATTR Value 1e-8
SYMBOL res 336 800 R90
WINDOW 0 -6 58 VBottom 0
WINDOW 3 36 57 VTop 0
SYMATTR InstName R11
SYMATTR Value 910
SYMBOL diode 192 800 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName D5
SYMATTR Value 1N4148
SYMBOL diode 128 944 R270
WINDOW 0 32 32 VTop 0
WINDOW 3 0 32 VBottom 0
SYMATTR InstName D6
SYMATTR Value 1N4148
SYMBOL res 336 912 R90
WINDOW 0 -11 56 VBottom 0
WINDOW 3 40 59 VTop 0
SYMATTR InstName R12
SYMATTR Value 8200
SYMBOL zener 1568 784 R90
WINDOW 0 -4 32 VBottom 0
WINDOW 3 36 32 VTop 0
SYMATTR InstName D3
SYMATTR Value BZX84C15L
SYMBOL zener 1232 688 R180
WINDOW 0 -41 29 Left 0
WINDOW 3 -77 -2 Left 0
SYMATTR InstName D4
SYMATTR Value DFLZ33
SYMBOL zener 1232 800 R180
WINDOW 0 -37 31 Left 0
WINDOW 3 -70 -4 Left 0
SYMATTR InstName D7
SYMATTR Value DFLZ33
SYMBOL zener 1232 496 R180
WINDOW 0 -41 29 Left 0
WINDOW 3 -69 62 Left 0
SYMATTR InstName D8
SYMATTR Value DFLZ33
SYMBOL zener 1232 992 R180
WINDOW 0 -41 29 Left 0
WINDOW 3 -77 -2 Left 0
SYMATTR InstName D9
SYMATTR Value DFLZ33
TEXT 1680 944 Left 0 ;LOAD
TEXT 104 1056 Left 0 !.tran .1

 

[John Fields]

Version 4 [ snip '

OK, OK, that's MUCH better John. I'll give you
a pass on that version. But I won't make your
ASCII drawing for you. I have work to do!

---
I have a better solution.

For those who want to follow the discussion but don't want to use
LTSPICE, I'll post PDFs of the schematics to abse. The last one
will be on its way momentarily.

 

[Fred Bloggs]

I liked Fred's circuit better, but the current sink
+ zener scheme as part of the flying gate-voltage
generator bothered me.

You can put that current source under ground referenced logic control to
keep it off under high duty conditions where Cbst works just fine. We're
only talking about supplying leakage here.

Also, neither circuit addresses Michael's issues,
which involves connecting two batteries and a third
power source, to the 30A output power bus. The sad
power-dissipating 30A diode-OR issue and its wiring,
etc., is still unsettled.

Are you talking about this?

Nope, not frequent switching! The application is that there are a
couple power sources, and each power source will be switched with a
FET. A logic circuit chooses which power source to use and turns on
that FET. The circuit will use that FET for probably 10-30 minutes and
then once it is drained it'll switch to a difference source. That was
one of the main issues I ran into when initially looking for a driver
for the FET. Now I'm alot more interested in seeing if it's possible
to roll my own driver.

Sounds like maybe a solar power system...This is another good reason for
going with a P-channel. Anyway, what's the matter with the usual MOSFET
blocking switch arrangement, one ckt for each power source, we're not
talking big bucks here:
View in a fixed-width font such as Courier.

..
.. HV
.. |
.. 12V>--+-|>|-+--------+-----+-|<|------. |-
.. | | | | | .--||
.. | | C | | | |>
.. | | .--B npn | | | |
.. | | | E | | | |
.. | [4.7K] | +-[51]-----------|-----+ |
.. | | | E | | | |
.. | '----+--B pnp | | | |
.. [1K] | C === | | |
.. | | | |Cbst | | |
.. | | '-----+----------------|-----+
.. | | | | | |
.. | |- +--+-|>|---+ | |>
.. +-------||SSN1N45B | | | '--||
.. | |> | | [100] |-
.. \| | | | | |
.. 2N7000 ||-------+ -/ | === +---> out
.. <| | 12VZ ^ ----- | |
.. | | | |C555 |--' -
.. --- | | |ASTBL| ^
.. sgnd | | |10KHz| |
.. | | ----- |
.. | | | ---
.. CMOS IN >----' +----' pgnd
.. |
.. /|\ 10mA
.. CMOS EN >------------ \V/
.. |
.. ---
.. sgnd
..
..
..

 

[Fred Bloggs]

I liked Fred's circuit better, but the current sink
+ zener scheme as part of the flying gate-voltage
generator bothered me.

You can put that current source under ground referenced logic control to
keep it off under high duty conditions where Cbst works just fine. We're
only talking about supplying leakage here.

Also, neither circuit addresses Michael's issues,
which involves connecting two batteries and a third
power source, to the 30A output power bus. The sad
power-dissipating 30A diode-OR issue and its wiring,
etc., is still unsettled.

Are you talking about this?

Nope, not frequent switching! The application is that there are a
couple power sources, and each power source will be switched with a
FET. A logic circuit chooses which power source to use and turns on
that FET. The circuit will use that FET for probably 10-30 minutes and
then once it is drained it'll switch to a difference source. That was
one of the main issues I ran into when initially looking for a driver
for the FET. Now I'm alot more interested in seeing if it's possible
to roll my own driver.

Sounds like maybe a solar power system...This is another good reason for
going with a P-channel. Anyway, what's the matter with the usual MOSFET
blocking switch arrangement, one ckt for each power source, we're not
talking big bucks here:
View in a fixed-width font such as Courier.

.
. HV
. |
. 12V>--+-|>|-+--------+-----+-|<|------. |-
. | | | | | .--||
. | | C | | | |>
. | | .--B npn | | | |
. | | | E | | | |
. | [4.7K] | +-[51]-----------|-----+ |
. | | | E | | | |
. | '----+--B pnp | | | |
. [1K] | C === | | |
. | | | |Cbst | | |
. | | '-----+----------------|-----+
. | | | | | |
. | |- +--+-|>|---+ | |>
. +-------||SSN1N45B | | | '--||
. | |> | | [100] |-
. \| | | | | |
. 2N7000 ||-------+ -/ | === +---> out
. <| | 12VZ ^ ----- | |
. | | | |C555 |--' -
. --- | | |ASTBL| ^
. sgnd | | |10KHz| |
. | | ----- |
. | | | ---
. CMOS IN >----' +----' pgnd
. |
. /|\ 10mA
. CMOS EN >------------ \V/
. |
. ---
. sgnd
.
.
.

Oops- lost my Cbst charge-up ckt:
View in a fixed-width font such as Courier.

..
.. HV
.. |
.. 12V>--+-|>|-+--------+-----+-|<|------. |-
.. | | | | | .--||
.. | | C | | | |>
.. | | .--B npn | | | |
.. | | | E | | | |
.. | [4.7K] | +-[51]-----------|-----+ |
.. | | | E | | | |
.. | '----+--B pnp | | | |
.. [1K] | C === | | |
.. | | | |Cbst | | | 5W
.. | | '-----+----------------|-----+--[1k]--.
.. | | | | | | |
.. | |- +--+-|>|---+ | |> ---
.. +-------||SSN1N45B | | | '--|| sgnd
.. | |> | | [100] |-
.. \| | | | | |
.. 2N7000 ||-------+ -/ | === +---> out
.. <| | 12VZ ^ ----- | |
.. | | | |C555 |--' -
.. --- | | |ASTBL| ^
.. sgnd | | |10KHz| |
.. | | ----- |
.. | | | ---
.. CMOS ON >----' +----' pgnd
.. |
.. /|\ 10mA
.. CMOS EN >------------ \V/
.. |
.. ---
.. sgnd
..
..
..

 

[Fred Bloggs]

Don't need all that:
View in a fixed-width font such as Courier.

..
.. HV
.. |
.. 12V>--+-|>|-+--------+-----+-|<|------. |-
.. | | | | | .--||
.. | | C | | | |>
.. | | .--B npn | | | |
.. | | | E | | | |
.. | [4.7K] | +-[51]-----------|-----+ |
.. | | | E | | | |
.. | '----+--B pnp |0.1u | | |
.. [1K] | C === | | |
.. | | | |Cbst | | | 1/2W
.. | | '-----+----------------|-----+--[10k]-.
.. | | | | | | |
.. | |- +--+-|>|---+ | |> ---
.. +-------||SSN1N45B | | | '--|| sgnd
.. | |> | | [100] |-
.. \| | | | | |
.. 2N7000 ||-------+ -/ | === +---> out
.. <| | 12VZ ^ ----- |0.01u |
.. | | | |C555 |--' -
.. --- | | |ASTBL| ^
.. sgnd | | |10KHz| |
.. | | ----- |
.. | | | ---
.. CMOS ON >----' +----' pgnd
.. |
.. /|\ 10mA
.. CMOS EN >------------ \V/
.. |
.. ---
.. sgnd
..
..
..

 

[Michael]

Version 4 [ snip '

OK, OK, that's MUCH better John. I'll give you
a pass on that version. But I won't make your
ASCII drawing for you. I have work to do!

---
I have a better solution.

For those who want to follow the discussion but don't want to use
LTSPICE, I'll post PDFs of the schematics to abse. The last one
will be on its way momentarily.

I'd like to weigh in on one thing: I actually prefer spice drawings
over all other possibilities. For me, it just makes things simpler as
I can really get a better feel for how things are working, as it is
easy to probe. I often will convert ascii drawings into spice drawings
for this purpose. I don't have access to ABSE (well, that I know
about, at least) - and it's also not archived, so I don't think it's
as good of a place to post schematics.

-Michael

 

[John Larkin]

Don't need all that:
View in a fixed-width font such as Courier.

.
. HV
. |
. 12V>--+-|>|-+--------+-----+-|<|------. |-
. | | | | | .--||
. | | C | | | |>
. | | .--B npn | | | |
. | | | E | | | |
. | [4.7K] | +-[51]-----------|-----+ |
. | | | E | | | |
. | '----+--B pnp |0.1u | | |
. [1K] | C === | | |
. | | | |Cbst | | | 1/2W
. | | '-----+----------------|-----+--[10k]-.
. | | | | | | |
. | |- +--+-|>|---+ | |> ---
. +-------||SSN1N45B | | | '--|| sgnd
. | |> | | [100] |-
. \| | | | | |
. 2N7000 ||-------+ -/ | === +---> out
. <| | 12VZ ^ ----- |0.01u |
. | | | |C555 |--' -
. --- | | |ASTBL| ^
. sgnd | | |10KHz| |
. | | ----- |
. | | | ---
. CMOS ON >----' +----' pgnd
. |
. /|\ 10mA
. CMOS EN >------------ \V/
. |
. ---
. sgnd
.
.
.

And here's a slightly simplified version. Add the PV isolator for DC
drive.

ftp://66.117.156.8/Gate_Driver.JPG

One could also use a dip dc/dc converter and the relay.

John

 

[Michael]

No. At least not very much. You'll find that dies mounted
in TO-220 packages have the same thermal resistance as the
same dies mounted in a much larger TO-247 package. And as
the smaller D2-PAK package. It's the die's area footprint
that matters.

So the die size affects how the transistor handles transient loads,
but am I right in thinking that the package (and how it is heatsinked)
would contribute alot to how the transistor handled sustained loads?

They conduct current either way if the gate voltage is high
enough over the source. If the reverse-direction current is
high enough the drop across Rds(on) will increase to the
point where the anti-parallel intrinsic diode also conducts.

Do FETs typically drive current as well from D to S as from S to D?
Looking at various N-FET datasheets they don't really touch on that.

To make a fully-bidirectional four-quadrant switch, that is
completely OFF in one state and completely ON in the other.

So the idea is that when both gates were high, both FETs would conduct
current, but when both gates are low, you would have two diodes facing
each other with both FETs off, providing no path for current?

When the FETs are both ON you get 2*Rds(on), not 1.3 volts.

So this is essentially my question from above - but would Rds(on) =
Rsd(on)?

You may be better off with a diode - assuming you want to
switch from one source to the other without a power break,
you'll need a diode-OR action. An ON/OFF two-MOSFET switch
could conduct huge currents from a high-voltage battery into
a low one.

Before leaving the topic, it's worth noting that MOSFETs
as active rectifiers are better and cheaper than Schottky
diodes. For example, a 60CTQ045 dual 45V Schottky will
drop about 500mV at 30A, whereas an IRF1405 55V MOSFET
will drop only 210mV at 30A (both warmed up to Tj = 100C).
The diode costs $2.78 qty 100, vs $2.64 for the MOSFET.

However, aren't MOSFETs not really rectifiers? I mean from what you're
saying, it sounds like FETs can handle current in both directions.

I'll answer later. Meanwhile look at Fred's ASCII drawing,
as a start. I'd change a few things, but the basic idea
is to use a capacitor to deliver the MOSFET operating
voltages, and transistor level-shift and driver stages.

Thanks,

-Michael

 

[Michael]

Don't need all that:
View in a fixed-width font such as Courier.

.
. HV
. |
. 12V>--+-|>|-+--------+-----+-|<|------. |-
. | | | | | .--||
. | | C | | | |>
. | | .--B npn | | | |
. | | | E | | | |
. | [4.7K] | +-[51]-----------|-----+ |
. | | | E | | | |
. | '----+--B pnp |0.1u | | |
. [1K] | C === | | |
. | | | |Cbst | | | 1/2W
. | | '-----+----------------|-----+--[10k]-.
. | | | | | | |
. | |- +--+-|>|---+ | |> ---
. +-------||SSN1N45B | | | '--|| sgnd
. | |> | | [100] |-
. \| | | | | |
. 2N7000 ||-------+ -/ | === +---> out
. <| | 12VZ ^ ----- |0.01u |
. | | | |C555 |--' -
. --- | | |ASTBL| ^
. sgnd | | |10KHz| |
. | | ----- |
. | | | ---
. CMOS ON >----' +----' pgnd
. |
. /|\ 10mA
. CMOS EN >------------ \V/
. |
. ---
. sgnd
.
.
.

And here's a slightly simplified version. Add the PV isolator for DC
drive.

ftp://66.117.156.8/Gate_Driver.JPG

One could also use a dip dc/dc converter and the relay.

John- Hide quoted text -

- Show quoted text -

I'm trying to avoid the use of relays though - they are large and I
would think that they would be susceptible to way more bouncing than
I'm comfortable with.

-Michael

 

[Michael]

Don't need all that:
View in a fixed-width font such as Courier.

.
. HV
. |
. 12V>--+-|>|-+--------+-----+-|<|------. |-
. | | | | | .--||
. | | C | | | |>
. | | .--B npn | | | |
. | | | E | | | |
. | [4.7K] | +-[51]-----------|-----+ |
. | | | E | | | |
. | '----+--B pnp |0.1u | | |
. [1K] | C === | | |
. | | | |Cbst | | | 1/2W
. | | '-----+----------------|-----+--[10k]-.
. | | | | | | |
. | |- +--+-|>|---+ | |> ---
. +-------||SSN1N45B | | | '--|| sgnd
. | |> | | [100] |-
. \| | | | | |
. 2N7000 ||-------+ -/ | === +---> out
. <| | 12VZ ^ ----- |0.01u |
. | | | |C555 |--' -
. --- | | |ASTBL| ^
. sgnd | | |10KHz| |
. | | ----- |
. | | | ---
. CMOS ON >----' +----' pgnd
. |
. /|\ 10mA
. CMOS EN >------------ \V/
. |
. ---
. sgnd
.
.
.

Hi Fred - I'm having trouble understanding your circuits. You use
symbols that I really am not familiar at all with, Could you please
tell me what this is:

/|\ 10mA
\V/

Current sink, maybe? Is the "CMOS EN" the enable for the current sink?

And is this one:


|
-/
12VZ ^

A 12V zener?

Lastly, what does "cbst" mean?

Thanks, and sorry if I'm being thick. All work and no sleep makes
Michael a dumb boy.

-Michael

 

[John Larkin]

Don't need all that:
View in a fixed-width font such as Courier.

.
. HV
. |
. 12V>--+-|>|-+--------+-----+-|<|------. |-
. | | | | | .--||
. | | C | | | |>
. | | .--B npn | | | |
. | | | E | | | |
. | [4.7K] | +-[51]-----------|-----+ |
. | | | E | | | |
. | '----+--B pnp |0.1u | | |
. [1K] | C === | | |
. | | | |Cbst | | | 1/2W
. | | '-----+----------------|-----+--[10k]-.
. | | | | | | |
. | |- +--+-|>|---+ | |> ---
. +-------||SSN1N45B | | | '--|| sgnd
. | |> | | [100] |-
. \| | | | | |
. 2N7000 ||-------+ -/ | === +---> out
. <| | 12VZ ^ ----- |0.01u |
. | | | |C555 |--' -
. --- | | |ASTBL| ^
. sgnd | | |10KHz| |
. | | ----- |
. | | | ---
. CMOS ON >----' +----' pgnd
. |
. /|\ 10mA
. CMOS EN >------------ \V/
. |
. ---
. sgnd
.
.
.

And here's a slightly simplified version. Add the PV isolator for DC
drive.

ftp://66.117.156.8/Gate_Driver.JPG

One could also use a dip dc/dc converter and the relay.

John- Hide quoted text -

- Show quoted text -

I'm trying to avoid the use of relays though - they are large

About the size of a sugar cube. An electronic driver circuit could
well be bigger.

and I
would think that they would be susceptible to way more bouncing than
I'm comfortable with.

Bounce won't matter; the initial contact hit charges the gate all the
way.

John

 

[Fred Bloggs]

Don't need all that:
View in a fixed-width font such as Courier.

.
. HV
. |
. 12V>--+-|>|-+--------+-----+-|<|------. |-
. | | | | | .--||
. | | C | | | |>
. | | .--B npn | | | |
. | | | E | | | |
. | [4.7K] | +-[51]-----------|-----+ |
. | | | E | | | |
. | '----+--B pnp |0.1u | | |
. [1K] | C === | | |
. | | | |Cbst | | | 1/2W
. | | '-----+----------------|-----+--[10k]-.
. | | | | | | |
. | |- +--+-|>|---+ | |> ---
. +-------||SSN1N45B | | | '--|| sgnd
. | |> | | [100] |-
. \| | | | | |
. 2N7000 ||-------+ -/ | === +---> out
. <| | 12VZ ^ ----- |0.01u |
. | | | |C555 |--' -
. --- | | |ASTBL| ^
. sgnd | | |10KHz| |
. | | ----- |
. | | | ---
. CMOS ON >----' +----' pgnd
. |
. /|\ 10mA
. CMOS EN >------------ \V/
. |
. ---
. sgnd
.
.
.

And here's a slightly simplified version. Add the PV isolator for DC
drive.

ftp://66.117.156.8/Gate_Driver.JPG

One could also use a dip dc/dc converter and the relay.

John

What makes that simpler? And actually, as long as we know the MOSFETs
can take fairly hefty transient peak power dissipation , I would prefer
to redo the input interface and stick an integrating capacitor in there
somewhere to slow the turn on/off to 15V/us range, especially now with
several of these circuits driving a common power bus, preventing
unrestrained slew rates from inducing transient currents into high
impedance nodes. Also, I would want to at least attempt a backup
interlock by connecting OUT to the gate of the 2N7000 through a resistor
divider, or something, keeping the switching off while a second power
source was on the power bus, but not turning itself off Then there's
fusing on the HV lead to consider as well as protection of the LV stuff
in the event of HV component failure, possibly UVLO and maybe thermal
shutdown and other things...

 

[Fred Bloggs]

Don't need all that:
View in a fixed-width font such as Courier.

.
. HV
. |
. 12V>--+-|>|-+--------+-----+-|<|------. |-
. | | | | | .--||
. | | C | | | |>
. | | .--B npn | | | |
. | | | E | | | |
. | [4.7K] | +-[51]-----------|-----+ |
. | | | E | | | |
. | '----+--B pnp |0.1u | | |
. [1K] | C === | | |
. | | | |Cbst | | | 1/2W
. | | '-----+----------------|-----+--[10k]-.
. | | | | | | |
. | |- +--+-|>|---+ | |> ---
. +-------||SSN1N45B | | | '--|| sgnd
. | |> | | [100] |-
. \| | | | | |
. 2N7000 ||-------+ -/ | === +---> out
. <| | 12VZ ^ ----- |0.01u |
. | | | |C555 |--' -
. --- | | |ASTBL| ^
. sgnd | | |10KHz| |
. | | ----- |
. | | | ---
. CMOS ON >----' +----' pgnd
. |
. /|\ 10mA
. CMOS EN >------------ \V/
. |
. ---
. sgnd
.
.
.

Hi Fred - I'm having trouble understanding your circuits. You use
symbols that I really am not familiar at all with, Could you please
tell me what this is:

/|\ 10mA
\V/

That's a symbol for a constant current source CCS. Could be something
like this:
View in a fixed-width font such as Courier.

 

[Winfield]

Winfield wrote:

So this is essentially my question from above - but would
Rds(on) = Rsd(on)?

If you're asking if the forward and reverse-current
Rds(on) values are the same, the answer is yes.

However, aren't MOSFETs not really rectifiers? I mean from what
you're saying, it sounds like FETs can handle current in both
directions.

Yes. Michael, you haven't told us what's allowed during
switching events. I've been assuming only one battery
should be delivering current at a time, and if you further
assume the current delivery should be uninterrupted, then
during switching you need two batteries enabled at once,
at least for a short overlap. But if simple MOSFET switches
are used they'll deliver current from the higher battery to
the lower, and this could be dangerously high current, up to
500A for an pair of IRF1405 FETs, looking at the datasheet
for Vgs = 12V. Hence a series diode to prevent this. This
diode, even if it's four 60CTQ045 sections (two diodes) in
parallel, suggested earlier, will dissipate another 10.5
watts, 17W total if added to 6.3W for the MOSFET switch.

 

[Fred Bloggs]

Yes. Michael, you haven't told us what's allowed during
switching events. I've been assuming only one battery
should be delivering current at a time, and if you further
assume the current delivery should be uninterrupted, then
during switching you need two batteries enabled at once,
at least for a short overlap.

Well he could put a high current inductor ( another $10) in series with
the load to coast him through a few usecs while he turns one switch off
and another on, then fuse the battery feeds in case something goes awry.

 

[Winfield]

Don't need all that:
View in a fixed-width font such as Courier.

. HV
. |
. 12V>--+-|>|-+--------+-----+-|<|------. |-
. | | | | | .--||
. | | C | | | |>
. | | .--B npn | | | |
. | | | E | | | |
. | [4.7K] | +-[51]-----------|-----+ |
. | | | E | | | |
. | '----+--B pnp |0.1u | | |
. [1K] | C === | | |
. | | | |Cbst | | | 1/2W
. | | '-----+----------------|-----+--[10k]-.
. | | | | | | |
. | |- +--+-|>|---+ | |> ---
. +-------||SSN1N45B | | | '--|| sgnd
. | |> | | [100] |-
. \| | | | | |
. 2N7000 ||-------+ -/ | === +---> out
. <| | 12VZ ^ ----- |0.01u |
. | | | |C555 |--' -
. --- | | |ASTBL| ^
. sgnd | | |10KHz| |
. | | ----- |
. | | | ---
. CMOS ON >----' +----' pgnd
. |
. /|\ 10mA
. CMOS EN >------------ \V/
. |
. ---
. sgnd

Right, we don't need all that. Let's switch the entire
battery + 12V, as John suggested, but let's do it with a
simple BJT totem-pole stage.

,----+-------, IRF1405
| | | MOSFET (4) 60CTQ045 sections
12V / 68k | ,--|<|--, paralleled Schottkies
| \ | | |
bat ---+--- / ---- | ----+-, ,-+-+-+----|>|--- to +HV bus
to +55V \ | | v | |
| |/ ------- |
+-----| --- _|_
| |\v | _\_/_, 15V
| | npn |_| ' | zener
| | | \ |
+--|<|--+---------+--\--'
| \
| ferrite bead
Enable |
3.3k |/
--/\/\---| 2n5551 (2)
|\v 160V npn
|
gnd

Might be a good idea to add a ferrite bead on the MOSFET's
gate to be sure it can't engage in RF oscillation while
slowly switching.

There is an up to 1mA drain on the battery when it's off,
but I assume that's OK.

 

[John Larkin]

Don't need all that:
View in a fixed-width font such as Courier.

.
. HV
. |
. 12V>--+-|>|-+--------+-----+-|<|------. |-
. | | | | | .--||
. | | C | | | |>
. | | .--B npn | | | |
. | | | E | | | |
. | [4.7K] | +-[51]-----------|-----+ |
. | | | E | | | |
. | '----+--B pnp |0.1u | | |
. [1K] | C === | | |
. | | | |Cbst | | | 1/2W
. | | '-----+----------------|-----+--[10k]-.
. | | | | | | |
. | |- +--+-|>|---+ | |> ---
. +-------||SSN1N45B | | | '--|| sgnd
. | |> | | [100] |-
. \| | | | | |
. 2N7000 ||-------+ -/ | === +---> out
. <| | 12VZ ^ ----- |0.01u |
. | | | |C555 |--' -
. --- | | |ASTBL| ^
. sgnd | | |10KHz| |
. | | ----- |
. | | | ---
. CMOS ON >----' +----' pgnd
. |
. /|\ 10mA
. CMOS EN >------------ \V/
. |
. ---
. sgnd
.
.
.

And here's a slightly simplified version. Add the PV isolator for DC
drive.

ftp://66.117.156.8/Gate_Driver.JPG

One could also use a dip dc/dc converter and the relay.

John

What makes that simpler?

Um, having 1/5 as many parts? Being easy to analyze?

And actually, as long as we know the MOSFETs
can take fairly hefty transient peak power dissipation , I would prefer
to redo the input interface and stick an integrating capacitor in there
somewhere to slow the turn on/off to 15V/us range, especially now with
several of these circuits driving a common power bus, preventing
unrestrained slew rates from inducing transient currents into high
impedance nodes.

The gate resistor controls slew rate.

Also, I would want to at least attempt a backup
interlock by connecting OUT to the gate of the 2N7000 through a resistor
divider, or something, keeping the switching off while a second power
source was on the power bus, but not turning itself off Then there's
fusing on the HV lead to consider as well as protection of the LV stuff
in the event of HV component failure, possibly UVLO and maybe thermal
shutdown and other things...

More parts!

The dc/dc converter + relay is ideal for a one-off by someone who
doesn't want to do a lot of design and pcb fab. You could build one in
20 minutes, no simulation or debugging needed.

You could even use latching relays if that would help.

John

 

[Winfield Hill]

Right, we don't need all that. Let's switch the entire
battery + 12V, as John suggested, but let's do it with a
simple BJT totem-pole stage.

Oops, I forgot, heavy wire!

,----+-------, IRF1405
| | | MOSFET (4) 60CTQ045 sections
12V / 68k | ,--|<|--, paralleled Schottkies
| \ | | |
bat ===o=== / ==== | ====o=, ,-+=+=+===|>|==== to +HV bus
to +55V \ | | v | |
| | ------- |
| |/ --- _|_
+-----| | _\_/_, 15V
| |\v |_| ' | zener
| | npn | \ |
+--|<|--+---------+--\--'
| \
| ferrite bead
Enable |
3.3k |/
--/\/\---| 2n5551 (2)
|\v 160V npn
|
gnd

Might be a good idea to add a ferrite bead on the MOSFET's
gate to be sure it can't engage in RF oscillation while
slowly switching.

There is an up to 1mA drain on the battery when it's off,
but I assume that's OK.

Not that Michael needs it necessarily, but just to point out
that in general totem-pole circuits like this are attractive
ways to do HV switching, but suffer from two issues concerning
the pullup resistor: (1) as the output nears the V+ rail, the
base- and gate-current drive decreases, slowing turn-on, and
(2) the resistor creates an unpleasant high current when the
output is low, and off. For example, 68k above only delivers
a measly 60uA to work when ON within 4 volts of the V+ rail,
but wastes 1mA and 70mW when OFF at the ground level.

My solution is to replace the pullup resistor with a current
source, made from my favorite LND150 Supertex depletion-mode
MOSFET. The LND150N3 has a TO-92 package (or use the LND250
in a sot-23, or LND150N8 in a sot-89 package). Every circuit
designer should have a some in their stock drawers; get them
from Mouser at 55 cents each.

68k LN150 LND150 1k
--/\/\-- ==> ---, ,---+-- or ---, ,---/\/\--+---
_|__|_ | _|__|_ |
---, | ---, |
|___| |_________|

The simple LND150 by itself gives you a 1 to 2mA current
source with a 0.5 to 450-volt compliance range. Adding a
resistor lets you reduce the current to 0.5mA, 0.1mA, etc.

If you want a good 1mA, etc., for MOSFET turn-ON at the
full plus rail, but a reduced or zero current for the
turned-OFF condition, to reduce battery consumption or
power dissipation. If needed we can implement that
feature with a shutoff resistor from the +5 supply:

1n4004 R = +5/1mA = 4.7k +HV
+5 --|>|---/\/\---, |
| 1k |/
+HV in --------, ,--+--/\/\--+-------|
_|__|_ S | |\v
LND150 ---, | |
| +---|<|---+-- MOSFET gate
'-----------+
|
|/
--/\/\--|
|\v
|
gnd

I haven't tried this, but it should do the trick.

 

[Fred Bloggs]

Don't need all that:
View in a fixed-width font such as Courier.

. HV
. |
. 12V>--+-|>|-+--------+-----+-|<|------. |-
. | | | | | .--||
. | | C | | | |>
. | | .--B npn | | | |
. | | | E | | | |
. | [4.7K] | +-[51]-----------|-----+ |
. | | | E | | | |
. | '----+--B pnp |0.1u | | |
. [1K] | C === | | |
. | | | |Cbst | | | 1/2W
. | | '-----+----------------|-----+--[10k]-.
. | | | | | | |
. | |- +--+-|>|---+ | |> ---
. +-------||SSN1N45B | | | '--|| sgnd
. | |> | | [100] |-
. \| | | | | |
. 2N7000 ||-------+ -/ | === +---> out
. <| | 12VZ ^ ----- |0.01u |
. | | | |C555 |--' -
. --- | | |ASTBL| ^
. sgnd | | |10KHz| |
. | | ----- |
. | | | ---
. CMOS ON >----' +----' pgnd
. |
. /|\ 10mA
. CMOS EN >------------ \V/
. |
. ---
. sgnd

Right, we don't need all that. Let's switch the entire
battery + 12V, as John suggested, but let's do it with a
simple BJT totem-pole stage.

,----+-------, IRF1405
| | | MOSFET (4) 60CTQ045 sections
12V / 68k | ,--|<|--, paralleled Schottkies
| \ | | |
bat ---+--- / ---- | ----+-, ,-+-+-+----|>|--- to +HV bus
to +55V \ | | v | |
| |/ ------- |
+-----| --- _|_
| |\v | _\_/_, 15V
| | npn |_| ' | zener
| | | \ |
+--|<|--+---------+--\--'
| \
| ferrite bead
Enable |
3.3k |/
--/\/\---| 2n5551 (2)
|\v 160V npn
|
gnd

Might be a good idea to add a ferrite bead on the MOSFET's
gate to be sure it can't engage in RF oscillation while
slowly switching.

There is an up to 1mA drain on the battery when it's off,
but I assume that's OK.

Well of course that's going to look simplified since you collapsed all
that boost generator stuff into a 12V battery symbol...and it needs to
be duplicated with each driver.
I found this self-boost charge pump topology for high side drive in
integrated circuit form:
View in a fixed-width font such as Courier.

..
.. HV
.. |
.. D1 D2 |-
.. 12.6V>----+-----|>|--+-----------+-|>|--+----- .-||
.. | | | |+ | | |>
.. | | [Rpu] === DRIVER--+ |
.. | | | C2 | | | | |
.. | | .-------------+-----+-----|----+
.. | | | | | | |
.. | |+ -| | | | |
.. | C1=== Q2 ||---+ | | |
.. | | <| | | | |
.. | | | | | | |
.. | '----+ | | | |
.. | | | | | |>
.. | - | | '-||
.. | D3 v | | |-
.. | - | | |
.. | | | | +---> out
.. +-+LMC555 +------' | |
.. | | | | -
.. | | 10KHz |- IN>---' ^
.. | |----------|| Q1 |
.. | | |> |
.. | | | ---
.. +---+ --- pgnd
.. | sgnd
.. ---
.. sgnd
..
..
.. Q1,Q2 SSN1N45B
..
..
..